Cellulosic fibrous webs, particularly such webs associated with the manufacture of paper products, are formed by dispensing fibers randomly from a distributor or forming header onto a moving foraminous support means, generally a Fourdrinier wire. Three distinct methods of dispensing the fibers are in use currently. In the wet laid method, an aqueous slurry of said fibers is dispensed onto the wire, while in a variation of this process, an aqueous foamed dispersion comprising a high percentage of air by volume is dispensed. In the third general method, which method is the subject of the present invention, dry fibers are transported pneumatically to the forming header and dispensed onto the wire.
The fibers used in dry forming the web are first defiberized to proper freeness from pulp rolls, laps or bales, generally in a hammermill or equivalent defiberizing device. During this step mechanical energy from the rotors is converted into heat energy, the heat being transferred to the fibers whereby their moisture content is reduced appreciably. Low fiber moisture content compounds the difficulty of obtaining good quality product because the over-dry fibers tend to cling together as clumps while airborne due to electric static charges thereupon. These static charges are also undesirable because of the explosion dangers they pose. Finally, low and uncontrolled fiber moisture content is known to compromise the formation quality and caliper uniformity of webs obtained by the dry-laid process.
Two methods of overcoming the inherent problem of low fiber moisture, alone or in combination, have been used. The first method provides for pretreatment of the fibers with chemical antistatic agents or water before difiberization, but at high energy related costs and at the expense of product quality. The second method contemplates control of the humidity in the forming environment, typically a confined room. This second method is expensive because large volumes of air must be pretreated and distributed within the room with large lag times between set point humidity and the actual humidity therein.
Hence, there is need for a cost efficient easily controllable method for not only preventing excessive moisture loss of the fibers, but also to ensure an optimum moisture content.